.TH MPI_Cancel 3 "6/7/2023" " " "MPI"
.SH NAME
MPI_Cancel \-  Cancels a communication request 
.SH SYNOPSIS
.nf
.fi
.nf
int MPI_Cancel(MPI_Request *request)
.fi


.SH INPUT PARAMETERS
.PD 0
.TP
.B request 
- communication request (handle)
.PD 1

.SH NOTES
The primary expected use of 
.I MPI_Cancel
is in multi-buffering
schemes, where speculative 
.I MPI_Irecvs
are made.  When the computation
completes, some of these receive requests may remain; using 
.I MPI_Cancel
allows
the user to cancel these unsatisfied requests.

Cancelling a send operation is much more difficult, in large part because the
send will usually be at least partially complete (the information on the tag,
size, and source are usually sent immediately to the destination).
Users are
advised that cancelling a send, while a local operation (as defined by the MPI
standard), is likely to be expensive (usually generating one or more internal
messages).

.SH NULL HANDLES
The MPI 1.1 specification, in the section on opaque objects, explicitly
disallows freeing a null communicator.  The text from the standard is:
.nf
A null handle argument is an erroneous IN argument in MPI calls, unless an
exception is explicitly stated in the text that defines the function. Such
exception is allowed for handles to request objects in Wait and Test calls
(sections Communication Completion and Multiple Completions ). Otherwise, a
null handle can only be passed to a function that allocates a new object and
returns a reference to it in the handle.
.fi


.SH THREAD AND INTERRUPT SAFETY

This routine is thread-safe.  This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks.  However, the routine is not interrupt safe.  Typically,
this is due to the use of memory allocation routines such as 
.I malloc
or other non-MPICH runtime routines that are themselves not interrupt-safe.

.SH NOTES FOR FORTRAN
All MPI routines in Fortran (except for 
.I MPI_WTIME
and 
.I MPI_WTICK
) have
an additional argument 
.I ierr
at the end of the argument list.  
.I ierr
is an integer and has the same meaning as the return value of the routine
in C.  In Fortran, MPI routines are subroutines, and are invoked with the
.I call
statement.

All MPI objects (e.g., 
.I MPI_Datatype
, 
.I MPI_Comm
) are of type 
.I INTEGER
in Fortran.

.SH ERRORS

All MPI routines (except 
.I MPI_Wtime
and 
.I MPI_Wtick
) return an error value;
C routines as the value of the function and Fortran routines in the last
argument.  Before the value is returned, the current MPI error handler is
called.  By default, this error handler aborts the MPI job.  The error handler
may be changed with 
.I MPI_Comm_set_errhandler
(for communicators),
.I MPI_File_set_errhandler
(for files), and 
.I MPI_Win_set_errhandler
(for
RMA windows).  The MPI-1 routine 
.I MPI_Errhandler_set
may be used but
its use is deprecated.  The predefined error handler
.I MPI_ERRORS_RETURN
may be used to cause error values to be returned.
Note that MPI does 
.B not
guarantee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.

.PD 0
.TP
.B MPI_SUCCESS 
- No error; MPI routine completed successfully.
.PD 1
.PD 0
.TP
.B MPI_ERR_REQUEST 
- Invalid 
.I MPI_Request
\&.
Either null or, in the case of a
.I MPI_Start
or 
.I MPI_Startall
, not a persistent request.
.PD 1
.PD 0
.TP
.B MPI_ERR_OTHER 
- Other error; use 
.I MPI_Error_string
to get more information
about this error code. 
.PD 1

